While people still debate whether or not viruses can be considered life forms, some scientists, like University of Illinois researchers Arshan Nasir, Kyung Mo Kim and Gustavo Caetano-Anolles, propose that viruses get their own cherished spot on the tree of life. According to them, viruses deserve their own domain along with Archaea, Bacteria and Eukarya.
Just as archaeons have a different evolutionary history and biochemistry than bacteria (leading to them getting their own domain separate from bacteria) viruses also have a distinct history. In fact, the researchers suggest that viruses arose from a lineage that predated or coexisted with the earliest non-viral life forms on Earth. Much of this conclusion is based on studies of giant viruses. To be fair, not everyone agrees with the methodology used to make the case for a fourth domain. No doubt it will be debated for some time.
So what are giant viruses? As the name implies, these are extra large viruses that were first discovered two decades ago. How large? A typical giant virus has a genome with over a million bases and close to a thousand genes. ‘Regular’ viruses are far smaller. The influenza virus genome has between 12,000 and 15,000 bases and the HIV virus genome is less than ten thousand bases long. Meanwhile, one of the smallest bacteria, Mycoplasma genitalium, has a genome of less than 600,000 base pairs and only 521 genes. I should point out that no one disputes that tiny M. genitalium is alive. Intriguingly, some giant viruses have their own satellite viruses that act like parasites on the larger virus.
Megavirus particle.Thin section, electron microscopy by Chantal Abergel, 10/10/2011.
If you’re interested, the Giant Virus website maintains a top 100 list with the hundred largest viruses known to date. Currently holding the top position is Megavirus chilensis with a whopping 1.25 million bases and 1120 genes.